Select-low anti-lock brake control is a known anti-lock brake control. This control identifies one of right and left wheels that has a lower wheel speed and determines a control cycle that includes a decrease period, in which braking force on the right and left wheels is decreased, and an increase period, in which the braking force is increased, based on the slip amount (or slip rate) of the identified wheel.
Patent Document 1 discloses an example of a brake control device that performs select-low anti-lock brake control. When the driver applies the brakes, the controller determines based on the slip amounts of the right and left wheels whether the road surface on which the vehicle is traveling is a μ-split road surface, which refers to a road surface where the μ value of the road surface on which the left wheel is running is significantly different from the μ value of the road surface on which the right wheel is running.
Under the condition that the road surface is a μ-split road surface, when the slip amount of one of the right and left wheels that is running on the low μ surface (hereinafter also referred to as a low-μ side wheel) exceeds a first threshold, the anti-lock brake control is applied to the low-μ side wheel and also to the wheel running on the high μ surface (hereinafter also referred to as a high-μ side wheel). The braking forces applied to the low-μ side wheel and the high-μ side wheel are decreased in a decrease period, which is determined based on the slip amount of the low-μ side wheel, and increased in an increase period.
Further, in this brake control device, the tendency of the high-μ side wheel to lock is observed. Specifically, if the slip amount of the high-μ side wheel does not exceed a second threshold, which is less than the first threshold, in one control cycle including the previous increase period, the high-μ side wheel is determined to have less tendency to lock. Thus, the increase gradient of the braking force on the high-μ side wheel in the current increase period is set to be steeper than the increase gradient of the braking force on the high-μ side wheel in the previous increase period. If the slip amount of the high-μ side wheel exceeds the second threshold in one control cycle including the previous increase period, the high-μ side wheel is determined to have tendency to lock. Thus, the increase gradient of the braking force on the high-μ side wheel in the current increase period is set to be less steep than the increase gradient of the braking force on the high-μ side wheel in the previous increase period. This ensures the stability of behavior of the vehicle travelling on the μ-split road surface.